A free electron laser (FEL) provides powerful beams of laser light at low cost at continuously tunable frequencies by generating coherent optical radiation from a stream of relativistic electrons that move freely through a magnetic wiggler structure. FEL applications include industrial laser applications as well as military applications. For example, a FEL may be deployed in space or on a ship to defend against incoming missiles and rockets. Communications and power transmissions are other examples of uses for FELs.
One configuration of a FEL is at the Thomas Jefferson National Accelerator Facility, commonly referred to as Jefferson Lab or JLab, in Newport News, Va. The JLab FEL is based on a ring-architecture employing a single-axis Energy Recovery Linac (ERL). An radio frequency (RF) electron gun produces electrons and introduces them into multiple stages of superconducting RF linear accelerators (linacs) that produce electric fields which accelerate the electrons. The ring-based architecture also includes magnets that bend the electron trajectories enabling transport of the electron beam to the FEL gain generator and back to the entrance of the accelerator structures for energy recovery. The FEL can be configured as an oscillator consisting of a wiggler magnet structure and a resonator allowing photons to bounce back and forth to become a coherent laser. In other words, the FEL extracts energy from the electron beam and turns it into photon energy. Recirculation arcs may form part of the ring architecture to bend the electron beam through 360 degrees in order to inject the exhaust electron beam back through the same linac structure used to accelerate the pristine electron beam. The JLab FEL has demonstrated sustained operation at≈15 kW. However, progressing to high average power FELs requiring ampere-class electron beams significantly exacerbates the physics and engineering challenges that accompany such high current, high brightness electron beams for ring-based architectures.
Progression with high average power FELs has been slow due to technology challenges associated with management of the high average power electron and optical beams. The process of bending the electron beam degrades the electron beam phase space distribution which affects the gain out of the FEL and the ability to recover energy from the exhaust electron beam. If bending of the trajectory of the electron beam can be avoided, improved optical extraction efficiency can be achieved in addition to improved energy recovery.
Ring-based FEL architectures suffer efficiency impacts associated with the need to simultaneously accelerate a pristine beam and recover energy from an exhaust beam in a single-axis ERL structure principally due to challenges in exhaust beam transport and merge assemblies. FEL architectures that employ a single-axis ERL structure for both the acceleration of a pristine electron beam and energy recovery from an exhaust electron beam require the ERL structure to accommodate twice the average beam current seen by the FEL gain generator which limits total efficiency of the FEL device. Incorporating multiple photo-injectors into ring-based FEL architecture to overcome the required charge generation challenges for high average current devices is limited by the average current capacity of the single-axis accelerator structure.
It is with respect to these and other considerations that the disclosure herein is presented. All of the aforementioned features and performance impacts are mitigated in the proposed FEL system architecture.